A pyridoxalated hemoglobin-polyoxyethylene conjugate (PHP) is an oxygen carrier prepared from outdated human red cells through chemical modifications of hemoglobin (Hb). PHP was evaluated as a physiologic oxygen carrier. PHP was synthesized from a pyridoxalated hemoglobin (PLP-Hb) and an activated diester of polyoxyethylene (POE). The average numbers of molecules of PLP and POE conjugated to a Hb molecule were 3.3 and 6.2, respectively. The p50 of PHP was 20.4 torr, which was much higher than the 14.0 torr of stroma-free Hb. At a Hb concentration of 6 g per dL, PHP had a physiologic colloid osmotic pressure (29 torr) and a lower viscosity (2.6 cP at 37 degrees C) than whole blood (4.2 cP at 37 degrees C). At temperatures below 20 degrees C, the p50 of PHP was higher than that of the red cells. PHP, which had an average molecular weight of 90,000, consisted of 83 percent monomeric PLP-Hb-POE conjugate, 12 percent dimeric PLP-Hb-POE, and 4 percent trimeric PLP-Hb-POE. The consistency of the relative contents of the components was well controlled through seven lots, and each component had sufficient oxygen-carrying capacity. The dimeric and trimeric components contributed to the prolongation of the residence of PHP in the circulation. The results indicate that PHP has favorable properties as an oxygen-carrying blood substitute and perfusate.